Interest in solar cell technology has been increasing over the last years. Increasing energy costs as well as environmental concerns are factors behind this interest. Also technology breakthroughs, indicating the possibilities for large scale production of high efficiency solar cells have been important factors.
The most highly efficient existing solar cells are made of III-V semiconductors, such as GaInP or GaInAs, in multi junction cells with several layers each absorbing different parts of the solar spectrum. The advantage of this concept is illustrated by FIG. 1 showing the part of the solar AM1.5 spectrum that can be converted into electrical energy by a typical silicon photo voltaic (PV) cell compared to a GaInP/GaInAs/Ge tandem structure.
The theoretical limit for the power conversion efficiency of a solar cell based on a single semiconductor material is 31%. Multi junction photo voltaic cells (MJPV) can raise this limit to 43% for a dual junction and 49% for a triple junction solar cell. However, fabrication of all the necessary different material combinations is challenging and a high material quality of the crystals is essential for achieving high efficiencies.
Much progress has occurred and in December 2006 Boeing/Spectrolab announced (http://www.spectrolab.com/com/news/news-detail.asp?id=172) that they had demonstrated a record conversion efficiency of 40.7% using a 3-junction MJPV GaInP/GaInAs/Ge cells under 400× concentrated sunlight. This technology was, as mentioned in F. Dimroth, “High-efficiency solar cells from III-V compound semiconductors” Phys. Stat. Sol. (c) 3, 373 (2006), originally developed for space applications where Germanium (Ge) is a suitable substrate material. The availability of Ge in the Earth's crust is limited and it is expensive, and if such high efficiency tandem solar cells were used in large quantities on earth, this could be a limitation. For this reason, the development of multi junction solar cells based on crystalline Si, or even on simpler substrates, would open new opportunities for terrestrial applications, taking advantage of the higher multi junction efficiencies, lower cost and higher availability of Si substrates compared to Ge. A prior art multi junction photovoltaic cell comprising lattice matched layers grown on a Ge substrate is disclosed in L. L. Kazmerski “Solar photovoltaics R&D at the tipping point: A 2005 technology overview” J Electr Spectr Rel Phen 150, 105 (2006)). This MJPV cell reaches efficiencies of more than 40% with concentrators.
However, technical barriers for planar III-V multi junction solar cells can be identified. Efficiencies above 50% will be very difficult to reach due to physical limitations. Conventional III-V materials for multi junction solar cells require perfect lattice matching over large substrate areas to avoid dislocations. Good device functionality will also require a very high degree of compositional homogeneity over an entire wafer. This makes up-scaling to large area substrates extremely challenging, even if such substrates were available at reasonable cost. Even if these problems could be overcome, the limited number of materials that both have the right band gaps and are more or less lattice matched makes it very difficult to produce more than three junctions in planar solar cells, which is necessary for reaching very high efficiencies.
In addition to the above technical challenges, which are associated with the prior art multi junction cell, both cost and scaling present problems. By way of example multi-junction cells grown on Ge or III-V substrates are very expensive due to the high substrate costs and the small wafer sizes. Moreover, III-V materials are today epitaxially grown in high-grade MOCVD or even MBE reactors with low throughputs and the high cost of the precious raw materials makes the use of optical concentrators necessary to improve the cost-performance ratio on the system level. Even if the cost could be reduced, concentrators would still be necessary to achieve a saturated voltage even under full sunlight.